The members of the Circoviridae are the smallest autonomously replicating DNA viruses known and have been identified as pathogens in plants, avians and mammals. The form of virion DNA is unique as it is a single stranded circularized molecule of roughly 1800 bases. While the pathogenicity of the plant and avian circoviruses is well known, only recently have the disease potential of swine circoviruses been realized. The pathogenic potential of recently described human circoviruses remains to be determined. The long term goal of our multi-institutional and international research group is to elucidate pathogenic mechanisms of disease caused by infection with porcine circovirus type 2 (PCV-2), which can result in progression to fatal postweaning multisystemic wasting syndrome (PMWS) in swine. It is our central hypothesis that both pathogenic mechanisms and viral virulence determinants of PCV-2 can be elucidated by testing infectivity and pathogenicity of PCV-2 isolates and chimeric virus constructs in susceptible neonatal gnotobiotic swine inoculated with candidate viral variant(s). Specifically we will determine if unencapsidated single stranded (ss) circularized PCV-2 DNA occurs in serum of diseased pigs and if these DNAs are infectious for immunostimulated or immunosuppressed gnotobiotic swine and in gnotobiotic swine protected from infection by encapsidated virions by maternal-origin antibodies. Secondly, we will test the virulence potential of archival (ar) PCV-2, an infectious virus reconstructed from cloned viral DNA recovered from porcine tissues collected many years prior to the appearance of PMWS in swine. ArPCV-2 has been sequenced and a consistent change in nucleotide sequence, translating into a linear 3 amino acid sequence difference in the nucleocapsid protein between ar- and contemporary pathogenic (pa) PCV-2, has been found. It is our hypothesis that paPCV-2 acquired this change by mutation and this event may explain the emergence of pathogenic PCV-2 in recent years. Collectively, these data will provide critical insight into pathogenic mechanisms of disease mediated by mammalian circoviruses and will provide key guidance for the future study of human circovirus pathogens.